Hypogenic copper sulfides are an economically important sources of copper. Hypogenic deposits are formed by ascending solutions carrying high levels of metal ions at fairly high temperatures (up to 500° C.). As these solutions cool, metal sulfides are deposited as crystallized ore minerals as the solutions move up toward the Earth's surface. As a result, hypogenic deposits are characterized by metal sulfide bearing veins or irregular masses formed within fractures in the country rock. Within these hypogenic deposits, a variety of hypogenic copper sulfides may be found depending on the chemical composition of the ascending solution. Some of hypogenic copper sulfides found in hypogenic deposits are chalcopyrite, bornite, enargite, tetrahedrite, and tennatite. Hypogenic copper sulfides are also sometimes referred to as primary enriched copper sulfide minerals. The ascending solutions may also eventually reach the surface and appear as hot springs. In these situations, the solutions generally become diluted with ground water and thus have lower metal ion levels. As a result, the metal ions in these hot springs typically precipitate out as metal sulfate salts over time. In addition, the copper sulfide minerals that are formed above the water table may become altered over time by oxidation to sulfates by the circulation of air, water, and bacteria. These soluble metal salts are subsequently carried away in solution by the downward moving ground water. As the ground water moves to the oxygen deficient lower levels a secondary enrichment can take place. The copper-bearing solutions react with the existing chalcopyrite and other hypogenic sulfides such as bornite, enargite, tetrahedrite, and tennatite to form new copper sulfide minerals. The new minerals formed by the descending solutions are sometimes called supergenic or secondarily enriched copper sulfide minerals. The supergenic copper sulfides—or secondary enriched copper sulfides as they are sometimes referred to—are higher in copper and are characterized by the minerals covellite and chalcocite. They are also more readily oxidizeable copper sulfide minerals than the hypogenic copper sulfide minerals. These supergenic copper sulfide minerals are generally located below the oxidized zone and the water table and above the lower grade of primary sulfide ore.
Chalcopyrite is economically the most important hypogenic copper sulfide mineral, as well as the most economically important source of copper overall. Presently, smelting technology remains the primary technology for recovering copper from chalcopyrite. Smelting chalcopyrite, however, has a number of drawbacks. These include sulfur dioxide gas emissions which are environmentally unacceptable, large production of sulfuric acid even though there presently exist only a limited market for sulfuric acid in most areas, and expense. As a result, alternative methods for recovering copper from chalcopyrite, as well as other hypogenic copper sulfides, that are more environmentally friendly and less expensive have been sought for a number of years.
A number of alternatives that have been investigated for recovering copper from chalcopyrite and its ores have included hydrometallurgical processes. Hydrometallurgical processes have long been used to recover copper from oxide ores. These processes typically involve sulfuric acid leaching of the oxide ore, copper separation from the pregnant leach liquor by solvent extraction techniques and recovery of metallic copper from the strip liquor by electrowinning. These techniques have not only demonstrated an ability to recover copper at a competitive cost advantage over most smelting processes, but the electrowon copper produced in such processes is also now fully competitive in terms of quality with electrorefined copper produced by the known smelting and refining techniques. Presently, however, a commercially viable hydrometallurgical process for the recovery of copper from chalcopyrite, and other commercially important hypogenic copper sulfide minerals, has remained elusive despite extensive research efforts to develop such a process. The development of a hydrometallurgical process for the direct leaching of chalcopyrite either by chemical or biological means has been continuously sought for more than twenty years.
The direct leaching of chalcopyrite and other hypogenic copper sulfide minerals in sulfuric acid solution poses a variety of problems. At temperatures below the melting point of sulfur (approximately 118° C.), the rate of copper dissolution has, to date been uneconomically slow. At temperatures above the melting point of sulfur the chalcopyrite and other hypogenic copper sulfide minerals are passivated by what is believed to be a layer of elemental sulfur which forms over the unreacted sulfide particles. This again renders the extraction of copper uneconomical by this process. Other leaching systems that have been studied over the years for the extraction of copper from chalcopyrite on laboratory or pilot scale include systems employing concentrated solutions of ferric chloride or ammoniacal ammonium as lixiviants.
Efforts to bioleach chalcopyrite and other hypogenic copper sulfides on a commercial scale have also proven unsuccessful to date. Hypogenic copper sulfides such as chalcopyrite are notoriously difficult to bioleach even though bioleaching is now used as the principal production approach to extract copper from supergenic copper sulfide minerals such as chalcocite and covellite at several mining operations around the world.
Stirred tank and heap biooxidation processes that have employed mesophiles, such as Thiobacillus ferrooxidans, the most commonly used microorganism for biooxidizing sulfide minerals, have largely been unsuccessful due to the slow leach kinetics of chalcopyrite and other hypogenic copper sulfides. The slow leach kinetics and incomplete biooxidation of chalcopyrite and other hypogenic copper sulfides are often attributed to the formation of an inhibiting or passivation layer that forms on the surface of these copper sulfides as they oxidize. A number of different additives have been used in an attempt to increase the dissolution of copper from chalcopyrite, presumably by disrupting the passivating layer. These additives include metal salts such as Ag2SO4, Bi(NO3), graphite, and other sulfide minerals. Any biohydrometallurgical process for treating hypogenic copper sulfides such as chalcopyrite, therefore, will have to address the problem of this surface layer. Studies of the problem have led to several theories concerning the nature of the inhibiting layer.
One theory is that a jarosite coating forms on the surface of hypogenic copper sulfides as they are leached. Jarosite is formed in the presence of sulfate and ferric iron, in environments in which the pH increases to above about 1.8. However, high concentrations of jarosite constituent molecules (sulfate, ferric iron, ammonium or potassium) will lead to jarosite formation at lower pH. The presence of jarosite in analysis of bioleached chalcopyrite supports this theory. However, experiments performed by the present inventors that show slow leaching even at low constituent molecule concentration and low pH, as well as reports in the literature, contradict this theory.
Another theory is that elemental sulfur produced during bioleaching forms a thick blanket that excludes bacteria and chemical oxidants from the surface of the hypogenic copper sulfide minerals. The detection of large amounts of sulfur in bioleached chalcopyrite supports this theory. In addition, many electron micrographs have shown a thick sulfur coating on leached chalcopyrite. This theory, however, does not adequately explain why other metal sulfides that also form sulfur when leached do not leach as slow as chalcopyrite.
A third theory proposes that the inhibition is caused by the formation of an intermediate sulfide passivation layer. It is believed that this passivation layer is less reactive than the original hypogenic copper sulfide and may also inhibit the flow of electrons and oxidants to and from the hypogenic copper sulfide. The exact nature of this passivation layer is complex and is the subject of scientific debate. However, there is good agreement among the data in the literature that the passivation layer is unstable at higher temperatures. For example, it has been found that temperatures above about 60° C. are high enough to minimize the passivation of chalcopyrite during leaching.
Experiments with leaching at higher temperatures by both chemical and biological means have shown accelerated leaching of chalcopyrite. Chemical leaching done at over 100° C., however, requires expensive pressure reactors. Biological leaching is limited to the temperature limits of microorganisms that are capable of oxidizing metal sulfides or oxidizing ferrous to ferric. Some examples of microorganisms capable of oxidizing ferrous, metal sulfides, and elemental sulfur in environments above 60° C. include: Acidianus brierleyi, Acidianus infernus, Metallosphaera sedula, Sulfolobus acidocaldarius, Sulfolobus BC, and Sulfolobus metallicus. However, there are also other extreme thermophiles that can grow and leach metal sulfides at temperatures above about 60° C.
Stirred tank processes utilizing thermophiles have resulted in faster bioleaching of chalcopyrite than those using mesophiles have or moderate thermophiles have. Indeed, various microorganisms have been used in stirred tank processes to leach chalcopyrite concentrate in less than 10 days leaching time. However, the high temperature required for rapid leaching of chalcopyrite, as well as other hypogenic copper sulfides, increases the mass transfer limitations of oxygen and carbon dioxide in the system. This in turn has placed severe limitations on the pulp density that can be used in these stirred tank processes due to the high oxygen requirements of the thermophiles and the oxidation reaction occurring on the surface of the chalcopyrite during leaching. Thus, even though the bioleaching process can be completed in less than 10 days in a stirred tank process, the high operating and capital costs associated with operating a plant at the low pulp densities necessary to satisfy the oxygen requirements of the system have prevented the commercial implementation of stirred tank bioleaching for chalcopyrite concentrates, as well as for concentrates of other hypogenic copper sulfides.
If an effective heap bioleaching process could be developed for hypogenic copper sulfides, such as chalcopyrite, it would have the potential of operating at a lower cost than tank bioleaching of concentrate or pressure leaching of either concentrate or ores of hypogenic copper sulfides. Thus, heap leaching of hypogenic copper sulfides would be the preferred low cost procedure if a process could be developed to extract a high percentage of the copper in a matter of months. The use of thermophiles in a pilot scale heap leaching process is reported in Madsen, B. and Groves, R., Percolation Leaching of a Chalcopyrite-Bearing Ore at Ambient and Elevated Temperatures with Bacteria, 1983, Bureau of Mines. However, the process described in this paper was unable to achieve satisfactory recoveries in a reasonably short period of time and thus is not commercially viable. There have also been other reports of heap bioleaching processes reaching temperatures above 60° C. However, these too have not been commercially viable for extracting copper from chalcopyrite ores. The failings of all the reported heap bioleaching processes for chalcopyrite ores is that they have all generally taken over one year to leach and recover less than 50% of the copper in the chalcopyrite. The reasons for this are not entirely clear. However, the present inventors have determined that there are several factors that have acted together to prevent successful heap bioleaching of chalcopyrite ore. The first is that the heaps that have eventually reached a temperature of 60° C. or higher have taken a long time to build up enough heat to reach such high temperatures. As a result, once a temperature of 60° C. is reached, the amount of exposed sulfide mineral particles in the heap is insufficient to maintain the temperature to complete copper leaching. Furthermore, in the case of larger ore particles, such as those over about 2.5 cm, not enough of the copper sulfides in the ore are exposed to the leaching solution to permit adequate recoveries. Finally, the high temperatures can also increase the amount of ferric ion that precipitates as jarosite, which can further slow the leaching.